In the past, whipstocks have been used to create a window in a casing in a main wellbore for the initiation of a deviated wellbore which diverges from the original wellbore. After milling the window and drilling the deviated wellbore, a tubular is inserted through the window into the deviated wellbore. Prior completions have generally involved the absence of any cementing of the liner extending into the deviated wellbore or if cementing were done, it was terminated short of the window milled in the casing in the main wellbore. In those earlier techniques, since cement would not be allowed to come back from the deviated wellbore into the main wellbore, milling out the tubing from the main wellbore which extended into the deviated wellbore was not necessary if further production was required from the main wellbore. Completion techniques have evolved to the point where after milling the window and creating the deviated wellbore, the liner is placed through the window into the deviated wellbore and cemented. Thereafter, a milling operation is necessary to remove that portion of the liner in the main wellbore and to retrieve the whipstock, if it has not already been earlier retrieved.
FIGS. 1 and 2 indicate these procedures that had been previously required in view of the use of existing equipment, as just described. FIG. 1 illustrates a main wellbore 10 which has a deviated wellbore 12 already drilled through it. Inside of the main wellbore 10 is casing 14, which has already been milled through the use of a whipstock 16 and a milling tool (not shown). At the conclusion of the milling of the "window" 18 in the casing 14, a liner 20 is inserted into casing 14 and is diverted into window 18 into deviated wellbore 12, as shown in FIG. 1. In order to make the turn into deviated wellbore 12, the liner 20 winds up being wedged against one side of the casing 14, as shown in FIG. 1. Similarly, that same liner 20 in the deviated wellbore 12 also can become wedged against the uncased bore making up wellbore 12, as shown in FIG. 1.
After placement of the liner 20 into deviated wellbore 12, the deviated wellbore is cemented around the liner 20 up into the main wellbore 10. Thereafter, a milling tool 22 is employed to mill out the portion of the liner 20 that extends in wellbore 10. Thereafter, the whipstock 16 is removed. This procedure is illustrated in more detail in U.S. Pat. No. 5,301,760. FIG. 2 reflects the use of centralizers 24 in the deviated wellbore 12 to centralize the liner 20 therein. While the centralizers located in the deviated wellbore 12 help to centralize the liner 20, that portion of the liner 20 that extends into the wellbore 10 is still wedged firmly against the casing 14 within wellbore 10 due to the angular deflection of the liner 20.
There are many practical problems disclosed by the method in U.S. Pat. No. 5,301,760 that are not revealed in the patent. The biggest problem occurs when a milling tool such as 22 is employed to begin the milling operation. Typically, a "washover"-type milling tool is used which has cutting elements on the bottom and on the inside. This type of tool is called a washover tool because its purpose is to straddle the tubular object to be milled. This type of milling tool generally has no cutting elements on its exterior. Cutting elements on the exterior of the milling tool 22 would be undesirable since it would result in milling away of the wall of casing 14 in wellbore 10. The problem arises in the sense that with the liner 20 wedged up against the casing 14 in wellbore 10, the washover milling tool 22 cannot fully get around the upper end of the liner 20. Instead, as shown in FIG. 1, some milling goes on on the inboard side 26 of liner 20, while more complete milling takes place on the outboard side 28. The result of this uneven milling is that slivers are formed because segments of the inboard side 26 are not fully milled. Since segments of the inboard side are not fully milled, they retain additional structural strength which ultimately results in directing the milling tool 22 in a deviated path toward window 18. This is undesirable since continued milling with the milling tool 22 in a skewed or deviated position can result in unwanted milling of segment 30 of the casing 14, which is located below the window. FIG. 2 is intended to illustrate the formation of slivers and the skewing of the milling tool 22 when an attempt is made to use a washover tool over a liner 20 when the liner 20 is pressed rigidly against the casing 14. As a result of the milling process illustrated in FIG. 2, segment 32 while shown in the drawing is, in effect, fully milled away while only portions of segment 34 on the inboard side 26 is effectively milled due to the inability of the washover tool 22 to fully wash over the inboard side 26. Accordingly, as a result of the milling operation illustrated in the U.S. Pat. No. 5,301,760, slivers 36 are formed which subsequently must be fished out or further ground up before the whipstock 16 can be removed.
Accordingly, one of the several objectives of the method of the present invention is to facilitate the milling operation by providing mechanisms to keep the liner 20 away from the wall of the casing 14 to facilitate the washover milling operation. Additionally, it is a further object of the invention to facilitate the insertion of the liner through the use of a diverter. It is another object to employ a diverter of suitable dimensions to allow the washover mill to straddle it and ultimately latch into it so that any segments of cement or similar material or metal slivers which may be formed are caught within the washover tool when it is latched to the diverter. Ultimately, it is another object of the invention to be able to remove the diverter and thus provide a clear access to a packer which is further downhole in the main wellbore.